Memory devices can be categorized in two broad areas: volatile and non-volatile. Volatile memory devices require power to maintain data, while non-volatile memories are capable of maintaining data in the absence of a power supply. An example of a non-volatile memory is the flash memory that stores data in a semiconductor device without the need for power to maintain the data in the chip. A flash memory device stores data in numerous memory cells, which are usually formed in a semiconductor chip. Each of the memory cells often has a metal-oxide semiconductor (MOS) transistor with two different transistor gates: a control gate and a floating gate. The control gate may be used to turn the transistor on and off to control access to the memory cell. The floating gate may be the place where data is stored in each memory cell.
The data stored in the floating gate may correspond to the amount of electrons or charge in the floating gate. For example, the data stored in the floating gate may represent a first state (e.g., logic zero or binary 0 value) when an excess number of electrons is present in the floating gate and a second state (e.g., logic one or binary 1 value) when the excess number of electrons is absent from the floating gate. The presence or absence of the excess number of electrons in the floating gate may be controlled by varying the number of electrons in the floating gate, for example, by either adding electrons to or extracting electrons from the floating gate.
A programming operation (which is sometimes referred to as a write operation) may be used to add electrons to the floating gate and an erase operation may be used to extract electrons from the floating gate. Data in the memory cells may be read in a read operation. Programming, read, and erase operations in a conventional flash memory device usually involve applying voltages to the memory cells, such as to the control gates of the transistors and to other device components within the flash memory device.
Flash memory may be built using NOR or NAND devices. NAND flash may be of single-level cell (SLC) or multiple-level cell (MLC) configuration. MLC NAND flash allows for a higher density memory device in comparison to SLC NAND flash because it allows the storage of comparatively more data in each memory cell (e.g., two bits of data instead of just a single bit of data).
A conventional flash memory device may go through many programming, read, and erase operations during its life. Improper control of the voltage applied to the memory cells during these operations may lead to inferior device performance, reliability, or both. Thus, there is a need for improved apparatus, systems, and methods to assist in, for example regulating the voltages applied to memory cells.